In the primary cluture of chromaffin cells from cow adrenal medulla TH induction is preceded by an activation of cytosol cAPK and by an increase in nuclear protein phosphorylation. Both the induction of TH and the increase of nuclear phosphorylation require that the assembly of microtubular proteins be functional. Antimicrotubular drugs such as colchicine and vinblastine (10 to the minus 9th power M) can block the TH induction elicited by 8-Br-cAMP, when the drugs are added less than 15 hrs after 8-Br-cAMP. Since colchicine, added within cAMP also prevents the increase in nuclear phosphorylation, it is possible that the assembly of microtubular proteins might be operative in the intracellular translocation and nuclear uptake of catalytic subunits of cAPK activated by the addition of 8-Br-cAMP. In addition, these data support the view that an increase in nuclear protein phosphorylation is an essential step in the mediation of the acceleration of mRNA synthesis and the successive increase in TH synthesis elicited by 8-Br-cAMP.